Surface acoustic waves (i.e., "SAWs") also known as Rayleigh waves, have been known since the middle of the nineteenth century. However, it was not until much later that the phenomenon of SAW propagation was first exploited for its applications to electronic devices. Acoustic wave devices known in the art commonly consist of a substrate on which a conductive material is deposited in a predetermined pattern. The patterned conductive material is known as an interdigital transducer (i.e., an IDT). R. M. White et al., Appl. Phys. Let., Volume 7, Number 12, pages 314-316 (Dec. 15, 1965), describes the use of the IDT as an efficient technique for the generation and detection of surface acoustic waves on a piezoelectric surface. An IDT may be suitably connected to an electrical input so that the refractive index in a crystal is changed as required by acoustic-optic applications. See, e.g., K. S. Buritskii et al., Soy. Tech Phys. Lett. 17(8) pp. 563-565 (1991) and L. Hunn et al., Appl. Phys. Lett. 17(6) pp. 265-267 (1970). In other applications, an IDT on one end of a substrate surface may be connected to a source of the frequency waves (e.g., television antenna--radio frequency) and an IDT on the other end of the substrate surface may be connected to a device designed to receive a predetermined frequency (e.g., radio frequency for a specific television channel). The design of the IDT (i.e., the pattern of the conductive materials on the surface of a particular type of substrate) determines how the frequency will be controlled (e.g., which channel is received).
Frequency control devices using substrates capable of controlling the received frequency by the generation of acoustic waves known in the art are based on either bulk, i.e. unmodified, or modified crystalline material. For example, R. S. Wagers et al., IEEE Transactions on Sonics and Ultrasonics, Vol. SU-31, No. 3, pages 168-174 (May 1984) discloses acoustic wave devices based on bulk lithium niobate. Commonly assigned copending application Ser. No. 07/924,691, filed Jul. 31, 1992, now U.S. Pat. No. 5,350,961, discloses the use of bulk potassium titanyl phosphate (KTP) and surface modified domain reversed KTP in acoustic wave devices. K. S. Buritskii et al., Electronics Letters, Vol. 27, No. 21, pages 1896-1897 (Oct. 10, 1991), discusses the excitation of SAWs in Rb:KTP formed by Rb ion exchange on the entire surface of a single crystal of KTP). Buritskii et al., Sov. Tech. Phys. Lett., Volume 17, Number 8, pages 563-565 (August 1991) discusses the fabrication of a planar acousto-optic modulator using the Rb:KTP crystal.
The performance of frequency control devices based on acoustic waves is generally enhanced by providing some mechanism for guiding the acoustic waves. There are many types of acoustic waveguides currently available. For example, overlay waveguides employ a strip of one material placed on a substrate of another material. Substrate modified waveguides involve a local change produced in the properties of the substrate material (Acoustic Surface Waves, A. A. Oliner (Ed.) Springer-Verlag, P188, Ch 5, 1978). Acoustic waveguides of the substrate modified type have been made by ion-exchange processes using lithium niobate. The ion-exchanged region in the material will possess different physical properties from the unchanged region. The property variations due to the externally induced ions may include changes in mass density, optical refractive indices, changes in electromechanical coupling coefficients, and the variations in the acoustic velocities etc. (J. F. Weller, J. D. Crowley, and T. G. Giallorenzi, Appl. Phys. Lett., V 31, p 146-148, 1977). The larger the difference in physical properties between the ion-exchanged region and the non-ion-exchanged region, the better the waveguide confinement is. Due to the guiding effect, it is possible to enhance the device performance in acoustic beam focusing, beam reflection, and power density. In addition to the frequency control and signal processing applications, because of the tight confinement of the acoustic energy in the acoustic waveguide, the waveguide prevents the acoustic energy from leaking into overlay liquid such as water (R. L. Baer, C. A. Flory, M. Tom-Moy, and D. S. Solomon, IEEE Ultrasonics Symposium, p 293-298, 1992). Hence,the guiding property is very useful to convert acoustic wave devices from useless in liquid sensing into a highly sensitive sensing devices such as immunosensors. In wireless communication such as cellular phones, an important issue is to prevent the crosstalk (interference).
Acoustic waveguides also have great influences on devices in the optical signal processing applications such as acousto-optic tunable filters (AOTF) and acousto-optic modulators. The acousto-optic (A-0) modulators can be used in optical communications (D. A. Smith, and J. J. Johnson, IEEE Trans.on UFFC, V 40, p 22-25, 1993) and the acousto-optic tunable filters are useful in the entertainment, and analytical spectroscopy (X. Wang, Laser Focus World, May 1992). Since the performance of these devices is highly sensitive to the acoustic and light fields (Guided-Wave Acousto-Optics, Chen S. Tsai (Ed.), Springer-Verlag, 123-127, 1990), acoustic waveguides can provide a solution to the engineering optimization due to the abilities of increasing power density (more efficient), and controlling the optical phase.
The number of devices requiring frequency control has grown in number and complexity, and the demand for improved devices has grown commensurately.